1. Field of the Invention
The present invention relates to a liquid crystal display, more particularly, to a liquid crystal display employing a bend alignment made, improved in viewing angle property.
2. Description of the Related Art
A liquid crystal display (LCD) has significant advantages over a CRT (cathode ray tube), i.e., a smaller thickness, a lighter weight, and lower power consumption. A liquid crystal display generally comprises a liquid crystal cell and two polarizers provided on both sides of the liquid crystal cell. The liquid crystal cell generally comprises a rod-like liquid crystalline compound, two substrates for enclosing the compound, and an electrode layer for applying a voltage to the liquid-crystalline compound. An alignment layer is provided on each of the two substrates to align rod-like liquid crystalline molecules.
In order to reduce the coloration caused in images displayed on liquid crystal cells, optical compensation sheets (retardation plates) are often provided between the liquid crystal cells and polarizers. A combination of a polarizer and an optical compensation sheet serves as an elliptic polarizer. An optical compensation sheet is also used for improving the viewing angle of a liquid crystal cell.
Stretched synthetic polymer films (e.g., a polycarbonate film or polysulfone film) have been used as optical compensation sheets.
It has been also proposed to use an optical compensation sheet comprising a transparent support and an optically anisotropic layer formed of a liquid crystalline composition of a discotic compound instead of a synthetic polymer film. The optically anisotropic layer can be formed by aligning molecules of the discotic compound and fixing the alignment of the same. In general, discotic compounds exhibit a great birefringence. Further, discotic compounds have a variety of alignment modes. Therefore, the use of a discotic compound makes it possible to produce an optical compensation sheet exhibiting optical properties which cannot be obtained by a synthetic polymer film according to the related art. Various optical compensation sheets employing a discotic compound have been proposed (see JPA No. hei 6-214116, the term “JPA” as used herein means an “unexamined published Japanese patent application (Kohkai Tokkyo Kohou)”, U.S. Pat. Nos. 5,583,679 and 5,646,703, the publication of West Germany Unexamined Application No. 3911620).
A transparent support, supporting an optically anisotropic layer, is sometimes required to exhibit an optical isotropy (a low retardation value), and in such a case, a cellulose acetate film is usually used as a transparent support. A transparent support, supporting an optically anisotropic layer, is also sometimes required to exhibit an optical anisotropy (a high retardation value), and in such a case, a stretched synthetic polymer film (e.g., a polycarbonate film or polysulfone film) is usually used. In the technical field of optical materials such as optical compensation sheets, it has been a general rule to use a synthetic polymer film where a polymer film having optical anisotropy (a high retardation value) is required and to use a cellulose acetate film where optical isotropy (a low retardation value) is required. Overthrowing the general rule of the related art, a cellulose acetate film, which can be used even in applications where optical anisotropy is required, is disclosed in the publication of European Unexamined Patent Application No. 0911656.
Liquid crystal displays comprising a liquid crystal cell of employing a bend alignment mode, in which rod-like liquid crystalline molecules are aligned substantially in opposite directions (symmetrically) between an upper domain and a lower domain of the liquid crystal cell, are disclosed in the U.S. Pat. Nos. 4,583,825 and 5,410,422. Since the rod-like liquid crystalline molecules are aligned symmetrically between the upper domain and the lower domain of the liquid crystal cell, the bend alignment mode liquid crystal cell is capable of self-compensating optically. Therefore, the liquid crystal mode is also referred to as OCB (Optically Compensatory Bend) liquid crystal mode. A liquid crystal display employing a bend alignment mode is advantageous in that it has a high response speed. The bend alignment mode is characterized in that it provides a wider viewing angle and a higher response speed compared to other liquid crystal modes in the related art (TN mode and STN mode). However, the mode needs further improvement when compared to CRTs. A possible approach to further improvement of bend alignment mode liquid crystal displays is to use optical compensation sheets just as done in ordinary liquid crystal modes. Examples of the optical compensation sheets having an optically anisotropic layer formed of a composition comprising a discotic compound and examples of the bend-alignment mode LCD employing such sheets are disclosed in U.S. Pat. No. 5,805,253, the pamphlet of International Unexamined Patent Application No. 96/37804 (the publication of European Unexamined Patent Application No. 0783128), and U.S. Pat. No. 6,064,457.